Jaffa master in service to Apophis. Gar'tok supervised the instruction of several humans trained to one day infiltrate the Tau'ri. He showed Captains Rogers and Nelson a cave where weapons -- real weapons, not training intars -- would be used for the final challenge and eventually Earth's defeat.
Gar'tok then left the trainees to aid Apophis in a great battle, and was likely killed.
Rules of Engagement - Rogers tells of Gar'tok involvement in the training prior to his abandoning the team.